The present invention relates to a working fluid for traction drive or, more particularly, to a working fluid for traction drive comprising two kinds of specific compounds as the principal ingredients and capable of exhibiting excellent performance of traction drive.
A working fluid for traction drive generally means a fluid used in traction drive apparatuses, i.e. frictional drive apparatuses utilizing rolling contact, such as continuously variable transmissions for automobiles and industrial machines, hydraulic machines and the like. Working fluids for traction drive are required to have a high traction coefficient and stability against heat and oxidation in addition to inexpensiveness.
In recent years, studies on traction drive apparatuses are directed to the reduction of size and weight mainly in consideration of those mountable on automobiles. Correspondingly to this trend, the requirements for the working fluid for traction drive in these apparatuses are also escalating to have performance capable of withstanding various severe conditions under which the apparatuses are used. In particular, a working fluid for traction drive is required to exhibit high performance with stability over a wide temperature range from low temperatures, e.g. -30.degree. C., to high temperatures, e.g. 120.degree. C., including a high traction coefficient, relatively low viscosity, high oxidation stability and so on.
Various types of working fluids have been developed hitherto although none of them can satisfy all of the above mentioned requirements leaving problems in one or more respects. For example, a compound having a high traction coefficient as a working fluid at high temperatures usually has a high viscosity so that the efficiency of power transmission therewith is low due to the large agitation loss in addition to the problem in starting the traction drive apparatus at low temperatures. A compound having a relatively low viscosity and a high efficiency of power transmission, on the other hand, usually has a low traction coefficient at high temperatures and may cause troubles in the lubrication of the traction transmission apparatus due to the unduly decreased viscosity of the fluid at high temperatures.